Now, if $TMPDIR isn't set, it will create the socket in /tmp. Sometimes this folder is cleaned and you can loose access to your tmux server. If this happen, your process will still be running, but you simply won't be able to connect.

If you run tmux list-session you will receive an error message:

$ tmux list-session
failed to connect to server: Connection refused

When you check ps aux | grep -i tmux or top or htop you can definitely see that tmux is running. If you run lsof -U you can also see the socket that tmux has open.

Now, reading through tmux's man page, you can find the answer. Under -L it says:

If the socket is accidentally removed, the SIGUSR1 signal may be sent to the tmux server process to recreate it.